The present invention relates to a stepping motor of the inner rotor type having multiple stator tooth poles along an inner periphery of the motor, and relates to a stator yoke used in the stepping motor as well as a production method of the stator yoke.
FIG. 4 illustrates an example of the conventional stepping motor of the inner rotor type. A pair of interior yokes 2 (FIG. 8 or FIG. 9 shows a perspective view thereof) are coupled to each other at their bottom faces. Another pair of exterior yokes 1 (FIG. 7 shows a perspective view thereof), which support respective bearings 5, are coupled in opposed relation to the respective interior yokes 2 along their edge portions to form a stator yoke. A pair of bobbins 7 have wound coils 6 and are disposed around each phase of the stator yoke to thereby form an outer stator. A shaft 9 is fixed to a magnet 8 and to a pair of intermediate washers 10 to form an inner rotor, which is contained in a center of the stator.
FIGS. 5 and 6 show other examples of the conventional stepping motor of the inner rotor type. The FIG. 5 motor is different from the FIG. 4 motor in that this motor utilizes yokes of the type shown in FIGS. 8 or 9 for both of the exterior and interior yokes 1 and 2 instead of the exterior yokes shown in FIG. 7 or FIG. 10. Each exterior yoke 1 is fixed to a casing 3 of a cylindrical cup-shape. In similar manner, the FIG. 6 motor is different from the FIG. 4 moter in that this motor utilizes an exterior yoke 1 (FIG. 10 shows a perspective view thereof) which does not have a hole for receiving a bearing 5, and which is attached to a bearing support plate 4.
In making an exterior yoke as shown in FIG. 7, a sheet composed of a magnetic material is drawn to shape a cup to form a ring yoke 1h and a bottom plate 1g. The bottom plate 1g is processed to form tooth poles 1a.1b, corresponding openings 1i around the tooth poles and a center hole 1j prepared to receive therein a bearing. The tooth poles 1a .1b are raised vertically to the bottom plate 1g. The yoke is formed by the above described punching/raising method.
The FIG. 8 yoke is formed also by the punching/raising method such that tooth poles 1a.1b are punched in a center of a bottom plate 1g of a magnetic sheet and such that the tooth poles 1a.1b are raised vertically from the bottom plate 1g.
The FIG. 9 yoke is formed such that a sheet of a magnetic material is drawn to form a central convex and then tooth poles 1a.1b are punched and raised from a bottom plate 1g.
The FIG. 10 exterior yoke is formed such that a sheet of a magnetic material is drawn to form a cup to define a ring yoke 1h and a bottom plate 1g. Next, a center of the bottom plate 1g is again drawn oppositely and is die-cut to form tooth poles 1a 1b, which are then raised vertically relative to the bottom plate 1g.
For example, the punching/raising method is disclosed in FIG. 2 of prior art of Japanese patent application Laid-open No. 56-86054, and in FIG. 3 of Japanese utility model registration application Laid-open No. 56-133777. The drawing and punching method is disclosed in Japanese patent application Laid-open Nos. 56-86054 and 57-211964.
However, the conventional stepping motor has the following problems:
(1) As shown in FIG. 13, if a stator yoke has a relatively small inner diameter ID and a length L of each tooth pole is set to L&gt;ID/2, then an inner yoke cannot be produced by the punching/raising method since the tooth pole is raised from a center of the yoke. The drawing and punching method also has similar difficulty as the length L becomes greater than ID. In case of L&gt;&gt;ID/2, the production is impossible. For this reason, in the stepping motor of a small diameter, the stator yoke has also a small inner diameter so that the tooth pole cannot be elongated, hence the motor cannot be elongated axially, thereby failing to improve torque.
(2) As shown in FIG. 14(a), if the tooth pole is extensively elongated as compared to the FIG. 14(b) case, the amount of deformation is increased by the drawing work so that a plate thickness T1 of the tooth pole shown in FIG. 14(a) is smaller than a plate thickness T2 shown in FIG. 14(b). Namely, if L1&gt;L2, T1&lt;T2 is held. If such a yoke is used to constitute a magnetic circuit in the motor, there cannot be obtained an effective magnetic flux proportional to the length L1 of the tooth pole to thereby inclease a magnetic flux leakage. For this reason, there cannot be obtained an increase in torque even though the length of the tooth pole is extended.
(3) The drawing and punching method utilizes a die to punch the yoke (as shown in FIGS. 9 and 10). Since the sheet is drawn in a straight cylindrical shape or tapered cup shape along the punching direction, the cutting die has a long cutting stroke during the press-punching of the cylindrical convex, thereby reducing a life of the die. The yield rate of press-punched pieces is reduced for each die thereby increasing the production cost.
(4) With regard to the punching/raising method of raising tooth poles from a center of the yoke (shown in FIG. 8) and the drawing and punching method (as shown in FIGS. 9 and 10), when the yoke has a small inner diameter and therefore has narrow spaces, if a punch or die is dimensioned in a large size to secure the mechanical strength thereof, it is difficult to enlarge an area of each tooth pole and to increase a number of tooth poles, thereby causing drawbacks that the torque is not improved and a step angle is not reduced.